A fuel cell is an electrochemical device which reacts hydrogen from a fuel source, and oxygen, which is usually derived from the ambient air, to produce electricity, water, and heat. The basic process is highly efficient, and since electricity is created without combustion, most prior art types of fuel cells emit no or very reduced emissions as compared to other electricity generators. Further, since fuel cells can be assembled into modules of various sizes, power systems have been developed to produce a wide range of electrical power outputs. As a result of these attributes, fuel cell power systems hold a great deal of promise as an environmentally friendly and viable source of electricity for a great number of applications. One of a number of known fuel cell technologies is the proton exchange membrane (PEM) fuel cell. The fundamental electrochemical process under which PEM fuel cells operate is well understood and known in the art. The PEM is the preferred fuel cell technology for all but the largest power output applications given its low operating temperature, relatively low cost, and minimal requirements with respect to balance of plant devices or other assemblies which are utilized in combination with the fuel cell to facilitate the efficient operation of the fuel cell.
One of the greatest challenges facing the widespread implementation of PEM fuel cells relates to the source of fuel which is utilized with same. Most PEM fuel cells operate on essentially pure hydrogen gas, which is relatively expensive and sometimes difficult to secure as compared to infrastructure hydrocarbon fuels such as gasoline or natural gas. A great deal of effort is underway to develop a commercially viable fuel processor or reformer, which converts infrastructure fuels, such as those mentioned above, into gaseous hydrogen for use in fuel cells. Notwithstanding recent research efforts, these reformer technologies are still very expensive and complex, since they are essentially small-scale refineries.
Another approach to the fuel source problem described above is the development of PEM fuel cells that can operate directly on liquid hydrocarbon fuels, such as aqueous methanol, ethanol, or dimethyl ether. This type of fuel cell is commonly referred to as a “direct alcohol,” or sometimes more specifically, a “direct methanol” fuel cell, or “DMFC.” Direct alcohol fuel cells are typically proton exchange membrane (PEM) fuel cells that can accommodate an aqueous hydrocarbon fuel applied directly to the anode side of the membrane electrode assembly (MEA). In these arrangements, a noble metal catalyst (typically platinum) is embedded in the electrode and is able to utilize or otherwise extract a proton from the liquid fuel and then facilitate the reaction of the proton with the oxygen provided on the cathode side of the MEA, which is derived from ambient air.
Prior art PEM fuel cells have been configured in a traditional stack arrangement or in a planar arrangement heretofore. One such planar arrangement is the cartridge configuration as seen in U.S. Pat. Nos. 6,030,718 and 6,468,682, both of these teachings are hereby incorporated by reference. One possible stack arrangement is seen in U.S. patent application Ser. No. 11/800,994, the teachings of which are also incorporated by reference herein. These prior art configurations may also be applicable for direct liquid PEM fuel cells. One end-use application for direct liquid fuel cells that appears especially promising is the replacement of conventional batteries in portable electronic devices, such as cell phones, laptop computers, digital music players, and the like. Small-scale, portable fuel cells offer the promise of greater run times for these devices and less environmental impact than traditional batteries. Given their normally small size, direct alcohol fuel cells are generally configured in a planar configuration, as opposed to a conventional stack. These fuel cells operate at relatively low temperatures and without any active cooling systems and are thus suitable for small-scale portable electronics applications.
One primary challenge for direct liquid fuel cell designers is the even distribution of the liquid fuel mixture or solution across the anode side of the MEA. In larger PEM fuel cells, this fuel distribution is typically done with complex fuel distribution channels which are formed into either a bipolar separator plate or an adjacent gas diffusion layer. Further, it should be understood that with PEM fuel cells which are fueled by gaseous hydrogen, the fuel flows quite easily through the fuel distribution channels due to its small molecular size. However, liquid fuels as described above do not disperse through fuel distribution channels as readily as hydrogen gas. Further, it should be understood that the complexity of the fuel distribution schemes for larger PEM fuel cells is often difficult to apply to smaller-scale direct liquid fuel cells.
One proposed solution to this problem is disclosed in U.S. Pat. No. 6,497,975 to Bostaph et al., the teachings of which are hereby incorporated by reference. In the reference to Bostaph, fuel and exhaust distribution channels are formed within a multilayered base component that communicates with the MEA. These channels form a flow field that is integrated into the fuel cell body during the manufacture of the body, and these flow fields distribute the liquid fuel across the face of the PEM anode. A similar approach is disclosed in U.S. Pat. No. 7,071,121 to Punsalan et al., the teachings of which are also incorporated by reference herein. In the reference to Punsalan, the fuel and exhaust channels are created in a ceramic electrode layer through a masked etching process that is similar to the method used to etch silicon wafers in semiconductor manufacturing. As with the reference to Bostaph, the channels are used as flow fields to distribute liquid or gaseous fuel across the face of a fuel cell anode. The problem with these approaches is that they are merely the same approach which was taken in larger PEM fuel cells and applied to a much smaller device. Additionally, it should be recognized that the attendant costs and complexity of such devices are not eliminated by this approach. Further, it should be understood that the distribution of the fuel across the anode is limited to the arrangement of the flow field channels.
A direct liquid fuel cell which avoids the shortcomings attendant with the prior art devices and practices utilized heretofore is the subject matter of the present application.